The Moon

Earth has only one natural satellite, the Moon. The Moon orbits Earth in an elliptical orbit at about 36 800 km/h. Its orbital radius varies from 356 000 km to 407 000 km. The Moon is about a quarter the size of Earth, and it has just more than 1 per cent of the mass of Earth. The density of the Moon is only 3.34 g/cm3 compared to Earth's 5.52 g/cm3.

Figure 6.11 The Moon as seen from the Earth. (Photo: J. Wilkinson)

The Moon is the brightest object in the sky after the Sun. Unlike the Sun, which emits its own light and heat, the Moon only reflects sunlight. The Moon is also one of the most widely studied objects in the solar system. It has been studied with the naked eye, telescopes and spacecraft. To date, the Moon is the only body in the solar system to have been visited by humans; this occurred for the first time in 1969.

Some scientists believe that the Moon was a body formed at the same time as other planets in the solar system and gravitationally captured by the Earth. Others think it is a fragment torn out of Earth's mantle. Yet another theory is that the Earth-Moon pair could be a double planet.

Early views about the Moon

The presence of the Moon in our sky has captured human interest throughout history. The Moon is so large and close to Earth that some of its surface features are readily visible to the naked eye. For centuries people have thought that some of the features on the Moon looked like a human face looking down on them, and often talked about the 'man in the Moon'. People also thought that the Moon had a fairly smooth surface, until Galileo's telescope showed the surface was covered with many craters and mountains as well as plains.